Immobilization stress of awake spontaneously hypertensive rats (SHR) dramatically increases circulating catecholamines, but does not significantly elevate regional cerebral blood flow or damage cerebrovascular integrity. Autoregulation of cerebral blood flow must be sufficient during stress to prevent damage to the blood-brain barrier, and prevent significant brain uptake of catecholamines. Glucose utilization (GU), a measure of functional neuronal activity, was measured with 2-deoxy-D-glucose in sympathetic ganglia of Fischer-344 rats, and was found to increase between 12 and 24 months of age in the superior cervical ganglion. The increase may reflect a compensatory response to reduced Beta-receptor function in the elderly rat, and be related to increased concentrations of circulating catecholamines. Increased circulating catecholamines in the senescent rat may be released by paraganglia, which are extra-adrenal chromaffin tissue and proliferate between 24 and 33 months of age. It was demonstrated that they contain high concentrations of catecholamines. On the other hand, catecholamine fluorescence in single neuronal parikarya were shown to decrease with age in brain catecholamine-containing neurons (A12 neurons which regulate pituitary hormones), as well as in some peripheral sympathetic ganglia (hypogastric ganglion).